It is known in the art of stereolithography to perform rapid prototyping of product molds or the product itself. The stereolithographic approach, as is known, uses an ultraviolet laser to scan across and selectively polymerize a monomer (i.e., solidify a liquid plastic) to build up a prototype layer-by-layer and line-by-line from a predetermined model of a part. In particular, the laser is focused on a portion of a bath of liquid resin which causes the liquid to polymerize (or solidify) where the focal point of the laser contacts (i.e., is incident on) the liquid. This technique allows a part to be rapidly produced that would otherwise take a long time to make through a molding process.
It is also known to do rapid prototyping using an ultraviolet laser to perform selective laser sintering of a powder. Sintering, as is known, is a process in which the temperature of a powdered material is raised to its softening point by thermal heating with a laser, thereby causing the particles of the powder to fuse together in the heated region. The temperature level needed for sintering depends on the material being sintered; but the higher the temperature is, the quicker it sinters. For example, iron powder melts at 1500.degree. C. but will sinter at 1000.degree. C. if the powder remains at that temperature long enough.
In the sintering process, a laser beam at a substantially constant power level is incident on a powder bed and a lateral layer of the part is fabricated by repeated scanning of the laser beam in successive lines across a layer of powder until the entire layer has been scanned. The laser is turned on at points where the powder is to be sintered, otherwise, the laser is off. When one layer is complete, the surface of the sintering bed is lowered, another layer of powder is spread over the previous, now sintered layer, and the next layer is scanned. This process is repeated until the part is complete.
However, one problem with laser sintering is that sintered layers tend to curl due to a thermal gradient (temperature difference) that exists between the high-intensity, small diameter, focal point of the laser beam at the sintering location and the surrounding material.
One technique employed to obviate this problem is to heat up the entire bed of powder to some temperature less than the sintering temperature, thereby reducing the thermal gradient between the laser beam and the surrounding material. While this technique may work for some polymer powders, when metal or ceramic powders are used, the technique is much less successful because of the higher sintering and melting temperatures involved. First, it is difficult to maintain a uniform temperature across the powder bed. Secondly, if the powder is raised to approximately half the melting temperature, the powder will sinter on its own in a matter of hours. If the powder bed temperature is lower than half the melting temperature, this may not control the curling problem at all.
Thus, it would be desirable to devise a sintering system that does not require heating of the entire powder bed, yet at the same time reduces the curl of the sintered material.